Integrated circuits often contain volatile memory elements. Typical volatile memory elements are based on cross-coupled inverters (latches) and are used to store data. Each memory element can store a single bit of data. In integrated circuits such as programmable integrated circuits, volatile memory elements are used to store configuration data.
Memory elements are typically arranged in arrays. Data may be loaded into the memory elements of an array and may be read from the memory elements of the array using data lines. Once loaded, the output of a memory element in a programmable integrated circuit can serve as a static control signal that is applied to the gate of an associated pass transistor. Blocks of programmable logic can be customized by turning on and off appropriate pass transistors in this way.
To ensure satisfactory high-speed operation, it may be desirable to overdrive the pass transistors. In a typical overdrive scheme, pass transistors are controlled using larger voltage levels than are normally used in powering logic gates (i.e., overdrive voltages are applied to the gates of the pass transistors).
Overdrive voltages may have a maximum voltage level that is limited by a time dependent dielectric breakdown (TDDB) mechanism. TDDB is a charge injection mechanism by which a transistor's gate oxide layer breaks down when the gate of the transistor is driven at elevated voltage levels for extended periods of time. In general, a thinner gate oxide layer breaks down more readily than a thicker gate oxide layer. Pass transistors typically have a gate oxide layer with minimum thickness. As a result, the maximum overdrive voltage that may be applied to the pass transistors cannot be much higher than the voltage that is used to power the logic gates, thereby limiting the performance of the pass transistors.